A film-type polarizer is, also known as a sheet polarizer, generally an optical member capable of polarizing light in a specific direction, and widely used in display devices, such as liquid crystal displays, etc. For example, the film-type polarizer is widely used in a liquid crystal display device in order to selectively transmit only light of a specific direction of the light emitted from a backlight and the light passing through a liquid crystal layer, and to control polarization.
As the film-type polarizer, a polarizer using a stretched polyvinyl alcohol (PVA) film stained with a dichroic dye has been the most frequently used in the liquid crystal display devices, etc.
However, since the polarizer using the stretched PVA film shows poor heat resistance and moisture resistance, its optical performances may be reduced by water penetration or relaxation of polymers. For this reason, this polarizer is sandwiched between protection films such as TAC, etc. to be attached to the liquid crystal display device. Further, an adhesive film is needed to attach this polarizer to the liquid crystal display device, which makes the polarizer have an overall thickness of 70-150 μm.
Due to such thickness, poor heat resistance and moisture resistance of the polarizer, there have been actually limitations in application of the polarizer using the stretched PVA film. Therefore, it was difficult to apply the polarizer using the stretched PVA film to new fields which require application of the polarizer, for example, smarter shade, anti-reflection films for OLEDs, etc.
Meanwhile, in order to solve the problems of the polarizer using the stretched PVA film, it has been suggested and studied to use a dye-type polarizer which is in the form of a thin film or a film including a dye layer having an oriented dichroic dye as a main component. Such dye-type polarizer exhibits excellent degree of polarization due to the dichroic dye, which is oriented in the dye layer using an additional alignment film.
However, it was found that this dye-type polarizer also exhibits relatively poor heat resistance. For example, when the existing dye-type polarizer was exposed to a high temperature of about 80° C. to test its heat resistance, orientation of the dichroic dye and degree of polarization of the polarizer were found to be greatly reduced, which may be attributed to randomization of the dichroic dye orientation in the dye layer due to heat energy.
In order to solve this problem, it was tried that an oriented liquid crystal material (a liquid crystal polymer, etc.) is added as a host in the dye layer so as to provide a dye-type polarizer showing more stable orientation of dichroic dye and improved heat resistance. However, this trial was also unsuccessful, and thus there is a continuous demand for a dye-type polarizer that exhibits superior heat resistance to maintain excellent orientation of the dye layer and high degree of polarization when exposed to heat at a high temperature.